In a prior-art fitting of this type, a check valve is arranged in the drain pipe, and a plug valve that can be actuated via rods is arranged under the check valve below the point of entry to the riser channel of the riser pipe. If the plug valve is closed, the basin can be filled corresponding to the height of the riser pipe. However, when the plug valve is opened when the basin is filled, the water runs off through the drain pipe and the elbow trap connected to it. Finally, the check valve prevents dirt present in the drain from rising up into the basin. However, it is not possible to prevent the drain fittings from becoming soiled and clogged, so that the valves will no longer operate reliably. These drain fittings must therefore be cleaned from time to time. In the case of the prior-art fittings, this is expensive and requires trade skill.